Traffic signals have long been used to regulate the flow of traffic. Generally, traffic signals have relied on timers or vehicle sensors to determine when to change the phase of traffic signal lights, thereby signaling alternating directions of traffic to stop, and others to proceed.
Emergency vehicles, such as police cars, fire trucks and ambulances, are generally permitted to cross an intersection against a traffic signal. Emergency vehicles have typically depended on horns, sirens and flashing lights to alert other drivers approaching the intersection that an emergency vehicle intends to cross the intersection. However, due to hearing impairment, road noise, air conditioning, audio systems and other distractions, a driver of a vehicle approaching an intersection will often not be aware of the warning signal being emitted by an approaching emergency vehicle, thus resulting in a dangerous situation.
This problem was addressed in the commonly assigned U.S. Pat. No. 3,550,078 to Long, which is incorporated herein by reference. The Long patent discloses that as an emergency vehicle approaches an intersection, the emergency vehicle emits a preemption request comprised of a stream of light pulses occurring at a predetermined repetition rate. A photocell, which is part of a detector channel, receives the stream of light pulses emitted by the approaching emergency vehicle. An output of the detector channel is processed by a phase selector, which then issues a phase request to a traffic signal controller to change or hold green the traffic signal light that controls the emergency vehicle's approach to the intersection.
While the system disclosed by Long proved to be a commercial success, it became apparent that the system did not have adequate signal discrimination. In addition, the length of time during which the pulse request signal remained active after the termination of light pulses was not uniform and sometimes too short to allow safe transit of the emergency vehicle.
Commonly assigned U.S. Pat. No. 3,831,039 (Henschel), which is incorporated herein by reference, improves on the system disclosed in the Long patent by improved selectivity of low repetition rate light sources of gas discharge lamps, such as fluorescent lights, neon signs, and mercury vapor lights. Further, Henschel improves the discrimination between a series of equally spaced light pulses and a series of irregularly spaced light pulses such as lightning flashes.
In the system disclosed by Henschel, the stream of light pulses must have proper pulse separation and continue for a predetermined period of time. Also, once a preemption call is issued to the traffic signal controller, the preemption call must remain active for at least a predetermined time period. The discrimination circuit disclosed by Henschel provides an improvement over the discrimination circuit disclosed by Long and results in improved discrimination.
Although such systems contemplated that preemption systems would be used for emergency vehicles, it was desirable to use them with non-emergency vehicles such as buses and maintenance vehicles. It thus became necessary to differentiate between different types of emergency and non-emergency vehicles. The commonly assigned U.S. Pat. Nos. 4,162,477 (Munkberg) and 4,230,992 (Munkberg), which are incorporated herein by reference, disclose an optical traffic preemption system wherein different vehicles transmit preemption requests having different priority levels, and in which the signal controller can discriminate between requests of differing priority and give precedence to the higher priority signal. The optical emitter disclosed by Munkberg transmits light pulses at a variety of selected predetermined repetition rates, with the selected repetition rate indicative of a priority level.
Commonly assigned U.S. Pat. No. 4,734,881 (Klein and Oran) which is incorporated herein by reference, provides for performance of the optical preemption functions with logic based circuity replacing a large number of discrete and dedicated circuits. The microprocessor circuitry utilizes a windowing algorithm to validate that pulses of light were transmitted from a valid optical traffic preemption system emitter.
Commonly assigned U.S. Pat. No. 5,172,113 (Hamer) which is incorporated herein by reference, discloses a method of optically transmitting data from an optical emitter to a detector mounted along a traffic route used specifically to receive data or to an optical traffic preemption system located at an intersection. Hamer allows variable data to be transmitted in a stream of light pulses by interleaving data pulses between priority pulses. For example, an emergency vehicle can transmit data in a stream of light pulses from an optical emitter that can include an identification code that uniquely identifies the emitter, an offset code that causes a phase selector to create a traffic signal timing cycle offset, and an operation code that causes traffic signal lights to assume at least one phase. Further, an emitter can transmit setup information, for example a range setting code that causes a phase selector to set a threshold to which future optical transmissions will be compared. Phase selectors constructed in accordance with the Hamer disclosure are provided with a discrimination algorithm which is able to track a plurality of optical transmissions with each detector channel. Optical emitters as disclosed by Hamer are provided with a coincidence avoidance mechanism which causes overlapping optical transmission from separate optical emitters to drift apart. Hamer discloses an optical signal format that allows variable data to be transmitted, while maintaining compatibility with existing optical traffic preemption systems.
One problem with all of the above described optical systems is that they require a line-of-sight to the signal controller at the intersection due to the optical nature of the preemption signal. Thus, while they may work acceptably for road systems which follow a rectangular grid pattern, they suffer several disadvantages. For example, where approaches to an intersection are blocked from line-of-sight or follow an irregular, curved or abruptly angled pattern, optically-based systems are not effective because they require a line of sight to the receiver.
Radio based, as opposed to optically based systems, for traffic control preemption have also been developed. For example, U.S. Pat. No. 2,355,607 (Shepherd) describes radio communications systems for vehicular traffic control wherein a directional transmission and/or reception located at the intersection, or on the vehicle, provides traffic light control based on coded signals transmitted from emergency vehicles. However, the inherent lack of directional precision of the radio system causes numerous traffic lights positioned parallel to the direction of travel to be affected. This is a major disadvantage because such prior art radio transmitter systems may erroneously pre-empt signal lights which are not on the approach route of an on-coming vehicle demanding preemption.
Radio transmitter systems also suffer from range inaccuracies which may be caused by signal attenuation or reflection. For example, a building may block, reflect, or attenuate a radio frequency which is not a line-of-sight signal. Since radio transmitter systems typically use signal strength to estimate range, signal attenuation gives rise to inaccurate range estimates at the receiving intersection electronics. Adverse weather, such as precipitation or fog, may also adversely affect the range sensitivity of existing radio transmitter dependent systems.
Efforts to reinforce radio systems with additional control functions are disclosed in U.S. Pat. No. 4,443,783 (Mitchell) wherein a directional transmitter is located in the approaching vehicle with omnidirectional receivers at intersections and multiple frequencies, selected frequency combinations, and selected red and amber light combinations provide accommodation for inaccuracies. U.S. Pat. No. 4,573,049 (Orbeck) discloses two way communication of information on intersection preemption request and action.
A major drawback of radio transmitters is that while they do not require a line-of-sight approach, their inherent lack of directionality means that they may erroneously control a signal light which is not on the vehicle's route but which is proximate the route.
There is therefor a need for a traffic preemption system for locations where approaches to an intersection are not line-of-sight or where road systems do not follow a rectangular grid pattern. Such a system would desirably offer the following advantages: (1) discretion without the need for a strobe as used in optical systems; (2) immunity from weather effects on system range; and (3) capability for easy implementation in applications with curving or abruptly angled approaches.